This invention relates to sewing machines; more particularly, it relates to apparatus for controlling the presser foot of such machines.
Sewing machines are generally provided with presser bar mechanisms for maintaining the work material in contact with a feed dog, part of a feeding system for feeding a work material through the sewing machine. The presser bar mechanism generally includes a presser bar terminating in a presser foot which is in contact with the work material. The presser bar is usually spring loaded downwardly to maintain engagement between the work material and the feeding dog, and a lever is provided to elevate the presser bar and foot free of the work material so as to allow removal of the same or adjustment thereof as well as to facilitate insertion of a new work piece.
In a conventional arrangement, the presser foot is urged against the work material by one spring which may be adjusted to provide a light pressure or an extremely heavy pressure, depending upon the heaviness of the fabric and the absence or presence of hurdles comprising stitch joints which must be fed through the sewing machine. As an example, a heavy work material would require substantially more pressure to have a feed dog obtain a grip sufficient to feed the material through the sewing machine. In industrial sewing machines undergoing high speeds, in the neighborhood of 4000 to upwards of 10,000 stitches per minute, it frequently occurs that rising of the feed dogs to the work material for propulsion of the same in a feeding direction will cause the presser bar mechanism to be thrown upward against its spring and maintain a "float" out of actual contact with the work material. This characteristic necessitates increasing the spring loading to the point where this float does not occur. This characteristic has been observed to be due to high inertia parts which, according to physical laws, resist a change in state from being in motion to not being in motion and vice versa. A solution is, of course, to provide a low inertia system which is generally accomplished by having very light weight parts connected to the presser foot which parts are urged by a lightweight spring against the work material. In this low inertia presser system a main presser bar spring is not called into play until a fixed amount of motion has taken place. In the prior art, U.S. Pat. No. 3,863,580, of Feb. 4, 1975, there is an example of a low inertia pressure system which is adjustable to regulate the amount of travel permitted to the low inertia system before calling the main presser bar spring into operation. However, this prior art system is cumbersome and expensive to manufacture and is not readily retrofittable into existing sewing machines.
What is required is a low inertia presser system which is economical and retrofittable into an existing sewing machine and which may have its low inertia travel adjusted to suit the immediate sewing needs of the machine.